The iOS keyboard includes a key dedicated specifically to accessing the host of Emojis you can use to express your mood. Part of this interface includes a section for Emojis you use frequently, and while it can be useful, suggestions aren't always ideal.
With a new jailbreak tweak dubbed FavoriteEmojis by iOS developer Adeem Mawani, you can manually designate your favorite Emojis by choice; these are the ones that will begin populating the “Frequently Used."